This invention relates to cooling circuits for turbomachinery and, specifically to a cooling circuit that delivers cooling air to the stage 4 nozzle wheelspaces of a gas turbine.
Multi-stage turbines typically comprise axially spaced, rotatable wheels fixed to the turbine rotor, with buckets or blades mounted about the wheel hubs for rotation with the rotor. These wheels are axially separated on the rotor by spacer wheels that are radially aligned with fixed, annular arrays of stationary nozzles. Each row of buckets or blades forms a turbine xe2x80x9cstagexe2x80x9d such that, for a 4 stage turbine for example, stage 1 is closest to the turbine combustor, and stage 4 is farthest from the combustor.
Certain advanced multi-stage gas turbines are air cooled, steam cooled, or air and steam cooled. In one example, stages 1 and 2 are steam cooled; stage 3 is air cooled; and stage 4 is left uncooled. In this arrangement, it is necessary to purge the stage 4 nozzle wheelspaces, i.e., the spaces located on opposite sides of the stage 4 nozzles, in areas radially inward of the nozzle blades. These stage 4 nozzle wheelspaces are thus also known as the stage 3 aft wheelspace (SAWS) and the stage 4 forward wheelspace (4FWS).
Adequate cooling or purging of the stage 4 nozzle wheelspaces requires air from a higher pressure source to ensure adequate outflow of the purge air, thus preventing ingestion of hot combustion gases into the wheelspaces. High wheelspace temperatures can reduce the life of the turbine wheels and spacers, and thus measures need be taken to avoid ingestion of the hot combustion gases into these areas.
The specific problem to be solved is the delivery of cooling or purge air to the stage 4 nozzle wheelspaces with minimum cycle performance penalty. Typically, air is delivered through the nozzle to exit into the nozzle forward wheelspace. Some of this air flows through the interstage seal to purge the nozzle aft wheelspace. An alternate solution for this arrangement might be to bleed some of the rotor air into these wheelspaces. However, the air used by the turbine rotor is taken from a higher compressor stage than necessary to purge the wheelspaces, and this would result in a substantial performance penalty.
The stage 3 bucket, due to the complexities of advanced machine rotor steam delivery systems, is cooled using compressor mid stage extraction air delivered through the stage 3 nozzle. Due to the nature of this delivery system, the stage 3 forward wheelspace (3FWS) has excess flow. It is also at a higher pressure than the stage 3AWS due to the nature of a gas turbine flowpath. It would thus be desirable to use the additional flow and pressure available in the upstream, or stage 3FWS, to purge or cool the stage 4 nozzle wheelspaces (the 3AWS and the 4FWS).
This invention reduces the total amount of secondary flow required to purge the turbine rotor wheelspaces and cool the turbine buckets, thus improving gas turbine efficiency. This is done by using seal leakage air from the stage 3FWS and guiding it via the stage 3 bucket to cool and purge the stage 3AWS and stage 4FWS. This system has the additional benefit of allowing a simplified stage 4 nozzle and surrounding stator design, since no air need be passed through the nozzle to purge the adjacent stage 4 wheelspaces.
In the exemplary embodiment, this invention adds holes through the stage 3 bucket forward and aft coverplates (skirts) to allow a metered amount of air to flow through the bucket shank to the bucket aft wheelspace (3AWS) and thereby provide SAWS wheelspace cooling. This air then flows through the stage 4 nozzle interstage seal and purges the 4FWS. The bypass holes in the bucket shank allow for an accurate and controllable airflow from the 3FWS into the 3AWS. It will be appreciated that the number of stage 3 buckets provided with bypass holes will depend on the cooling requirements.
In its broader aspects, therefore, the present invention relates to a multi-stage turbine wherein at least one turbine wheel supports a row of buckets for rotation, and wherein the turbine wheel is located axially between first and second annular fixed arrays of nozzles, and including a cooling air circuit for purging a wheelspace between the turbine wheel and the second fixed annular array of nozzles comprising a flowpath through a shank portion of one or more buckets connecting a wheelspace between the turbine wheel and the first fixed annular array of nozzles with the wheelspace between the turbine wheel and the second fixed annular array of nozzles.
In another aspect, the invention relates to a method of purging forward and aft wheelspaces on opposite sides of an array of nozzles fixed on a diaphragm located axially between forward and aft turbine wheels mounted in a turbine rotor, wherein said fixed array of nozzles are supported in a diaphragm that is provided with first seal segments, in radial alignment with a spacer wheel between the forward and aft turbine wheels, comprising:
a) supplying air under pressure to a wheelspace forward of the forward turbine wheel;
b) bleeding part of the air under pressure through said forward turbine wheel to the forward wheelspace on one side of the fixed array of nozzles; and
c) permitting air in the forward wheelspace to pass between the diaphragm and the turbine rotor into the aft wheelspace.